1. Field of the Invention
The present invention relates to an air-fuel ratio-controlling apparatus for use with an internal combustion engine equipped with a canister for temporarily storing fuel vapor generated in the fuel tank, the apparatus acting to release the fuel vapor stored in the canister into the intake system of the engine along with air.
2. Description of Related Art
A prior art automotive internal combustion engine is typically equipped with a canister for temporarily storing fuel vapor generated inside the fuel tank. The canister is connected to the intake side of the internal combustion engine via a release passage. A flow control valve consisting, for example, of a solenoid valve is disposed in the release passage. Fuel vapor stored in the canister is released into the internal combustion engine along with air into the release passage according to the opening of the flow control valve.
The technique of this kind is known, for example, in Japanese Examined Patent Publication No. Hei 7-3211. In this technique, the concentration of the remaining oxygen in the exhaust gas is detected. The amount of feedback for correcting the air-fuel ratio is calculated from the result of the detection. When the amount of deviation of the feedback correction amount caused by the fuel vapor released into the internal combustion engine exceeds a given amount, the amount of purged fuel vapor and air (evaporated gas) from the canister is reduced. Therefore, if a rich evaporated gas is introduced, the air-fuel ratio can be controlled to a desired value.
The prior art technique described above produces the following problems. In a high-temperature environment or where a highly volatile fuel is held in the fuel tank, it is assumed that a rich evaporated gas is introduced into the canister. When the apparatus is just switched from a non-purging mode to a purging mode, a large amount of fuel contained in the evaporated gas is introduced into the internal combustion engine. That is, the purged fuel becomes excessive. After this transient time, there arises the possibility of reducing the purged flow rate. In this case, if the purged flow rate is controlled to a desired amount during the transient time between the non-purging mode and the purging mode, the purged flow rate is reduced down to 0 because of the stop of the purge. Then, the purged flow rate is again controlled to a desired amount. In particular, the state changes as given by Equation (1): EQU purged flow rate&gt;0.fwdarw.purged flow rate=0.fwdarw.purged flow rate&gt;0 . . . (1)
Thus, the purge is intermittently and repeatedly carried out. As a result, the air-fuel ratio is varied and disturbed greatly, thus presenting various problems such as deterioration of the quality of the exhaust gas.